At the present time, electrical logic devices, i.e., logic devices whose states are switched electrically, are pervasively used in applications such as computers, telecommunications switches, etc. However, much effort has been directed toward the development of optical logic devices, i.e., devices whose states are switched and read optically, in the hope that such devices could utilize the high spatial bandwidth of free space optics to connect large two dimensional arrays of optical logic devices. Such a configuration would make applications such as, e.g., parallel processing relatively straightforward and more easily implemented than with electrical logic devices.
Practical implementation of such devices requires that the arrays satisfy several conditions. The devices should, of course, have optical inputs and outputs, and it should be possible to toggle between two states. It would be further desirable that the arrays could be configured so that the outputs of one array could be used directly as inputs for another array. This configuration would permit the arrays to be optically cascadable.
Those skilled in the art have directed much effort toward the fabrication of such devices and arrays. For example, a p-i (MQW) -n structure exhibiting the quantum confined Stark effect (QCSE) has been developed. MQW is an acronym for multiple quantum well. This structure will be referred to as a SEED diode. A SEED diode connected in series with another element acting as a load forms a SEED which is an acronym for self-electro optic effect device. If the load is another reverse biased SEED diode, a symmetric SEED (S-SEED) is formed. The S-SEEDs can be easily fabricated in arrays. The reflectivity and the responsivity are functions of both the optical wavelength and the applied voltage due to the quantum confined Stark effect. These devices are now well known to those skilled in the art and need not be described in detail. See, for example, IEEE Journal of Quantum Electronics, QE-25, pp. 1928-1936, 1989 and IEEE Journal of Quantum Electronics, QE-21, pp. 1462-1476, 1985 for detailed descriptions of the device.
The operation of the device is important to an understanding of the invention and will be briefly described. S-SEEDs have typically been operated in a dual rail geometry at .lambda..sub.0, the exciton wavelength, where there is a positive feedback which leads to a bistable or latching device. The functionality of the device is a set-reset latch which performs simple Boolean logic functions with differential optical input signals. The device is first set to a known state and input beams applied. If the beams have sufficiently different intensities, the state of the device switches. Reading of the states is done by two high-intensity equal bias beams which yield differential output intensities for the two states.
While perfectly adequate for many applications, the switching energies and switching times of the operating mode of the S-SEED described are relatively high and long, respectively, for some applications. Lower switching energies and shorter switching times would be desirable. It would also be desirable to have an operating mode which automatically resets the device in each cycle.
Of course, other optical switches have been developed. See, e.g., U.S. Pat. No. 4,985,621 issued on Jan. 15, 1991 to Aull et. al. The switch described used a series connected photodiode and resonant tunneling diode to generate a control signal. The control signal was then amplified and the amplified signal applied to the modulator. The switch sacrifices some attributes of the S-SEED switch that are most useful in many applications.